If you use spaces in site names or page names, the spaces will be replaced with "%20" in the URL of each page. For instance, if you have a site named "My Site" that contains a page named "My Page," the URL will be formatted http://web.mac.com/YourMemberName/iWeb/My%20Site/My%20Page.
This happens because some special characters (like a space) are encoded according to the the standard Uniform Resource Locator (URL) specification.